A new derivative of homocysteine, its mixed disulfide with methyl mercaptan, S-(methylthio-L-homocysteine (L-SMETH) was synthesized and found to be cytotoxic to L1210 leukemia cells in culture at micromolar concentrations. The inhibition is markedly promoted by added cupric ion, but not by ions of other metals, is stereospecific, and is competitive with glutamine. For example, at 10 M each of L-SMETH and Cu++, complete growth inhibition was observed if cells were incubated in 1 mM glutamine, 50% inhibition at 2 mM glutamine and none at 4 mM glutamine. The inhibition is also completely relieved by cytidine in a non-competitive manner, but not by guanosine or uridine, indicating that the principal damage to the cellular economy resides in the amination of uridine to cytidine. This was confirmed by HPLC analysis of cell extracts, which showed a marked decrease in CTP with increases in the levels of UTP, GTP, and ATP. A major swelling of cells leading to lysis accompanies the inhibition and increases in DNA and protein per cell confirms this unbalanced growth. High concentrations of SMETH-copper which are more than adequate to completely inhibit growth when left in the medium do not kill cells if they are washed free of inhibitor after 4 hours. The SMETH-copper compound is not toxic, and at dosages and schedules attempted decreased morbidity and peritoneal disease but did not increase the life span of tumor- bearing mice. It thus may require more extensive dose and schedule evaluation as observed with other glutamine amido-transferase inhibitors. Both the chemical and biochemical bases for activity of the novel bio-reactive compound are presented.